It is certainly best not to use any chemicals while pregnant, but the mother's stress also affects the unborn baby. If using weed relieves her stress, it could have a net positive affect. That said, smoking every day seems like too much to me. As others have said, moderation is key.
I personally don't believe that occasional weed use will carry a significant risk for the fetus.
This is based on the limited research I have seen, my discussions with doctors and midwives about this issue, and my experiences with women smoking a LOT of weed while pregnant. Our wonderful midwife complained about her rasta clients who don't stop smoking because of the low birth weight that resulted, and stopped taking clients that refuse to stop smoking continuously. However, from her discussion, I got the feeling that even continuous smoking had a relatively small negative effect, and therefore I suspect that infrequent smoking would have very very small negative effects, if any. Also, using a vaporizer would make a lot of sense, since the limited oxygen intake associated with smoking seems to be what affects the baby the most, as the mother is providing oxygen for the baby. Vaping would take care of this problem.
Also want to say this: I would much, much rather my wife smoke weed occasionally while pregnant than smoke tobacco or drink alcohol.
Also this: if she feels like she NEEDS to smoke, and often, I would try to discourage her. She should be strong enough to handle the stresses of life without constant escape. If she feels like she would like to smoke, and occasionally, I would not discourage her. Intelligent and moderate use of weed is different than escapist addiction-based use.
Just my opinions.
In any case, best of luck to you and your wife during this marvelous adventure, which only starts with the pregnancy!